Wave signaling system



June 13, 1933. w 'A MacDoNALD 1,913,604

WAVE SIGNALING SYSTEM Original Filed May 25, 1928 2 Sheets-Sheet l 550 we, I500 K.C.

FfifaufNc-y INVENTOR ATTORNEYS Junell3, 1933. w. A. M cDONALD 1,913,604

WAVE SIGNALING SYSTEM 4 Original Filed May 25, 1928 {Sheds-Sheet 2 INVENTOR' A M/fiam Mariana/d mum ATTORNEY5 1 v The invention Patented June 13, 1933 UNITED STATES PATENT OFFICE WILLIAM in menoNALn, or LITTLE imcx, NEW YORK, Assrenoa To HAZELTINE CORPORATION, A CQRPORATION or DELAWARE WAVE SIGNALING sYsTEm Original application filed May 25, 1928, Serial No. 280,464, and in Australia September 26, 1928. Divided and this application filed December 14, 1932. Serial No. 647,151.

This invention relates to vacuum tuberadio receivers and has to do more particularly with receivers intended for broadcast recep-v tion and kindred purposes.

This application is a division of my copending application Serial No. 280,464, filed May 25, 1928. v

has two' primary objects,

namely; (1) to provide an arrangement, de-

void of "complication, whereby radio waves of all frequencies within a predetermined band (such as the-broadcastband) can, be

received with approximately uniform efiec tiveness; and (2) to provide an arrangement in which the severaltunable circuitsof a tuned multistage radio-frequency receiving "amplifier may be operated through a common tuning control without their mutual resonance being adversely affected to any material extent by reason of the assoclation therewith of the antenna circuitand to do this without resorting to any expedient -which, in itself, is objectionable. The suc-' cessful accom 'ilishment of these objects is a matter of cardinal importance in the evolution of broadcast receiver design because it provides at once the means of surmounting- What, heretofore, have been considered two major obstacles lying in the way of the development' of an entirely satisfactory receiving instrument embodying strictly uniceivers.

fied tuning controla feature which is now. almost universally demanded in broadcast recult is tuned to a fixed frequency of the order of the lowestfrequency of the tuning range, whereby the antenna circuit is inductively reactive throughout. the range. The

antenna circuit is coupled directly to the first variably tuned circuit, either electromagnetically or electrostatically, or both. If

The present; invention is concerned-only with vacuumthbe systems (more particularly radio receivers including tuned radiofrequency amplifiers) having antennas of the untunable or invariably tuned variety. IThat system in which facilities are provided for directly tuning the antenna to each particular frequency which it is desired to receive. The invention is applicable especially to systems (such as broadcast receiving systems) which are designed to operate over a comparat-ively wide band of frequencies, as for example, the I present broadcast frequency range. The most important application of vthe inventionl s n broadcast receivers comprising two or more stages of tuned, (that is variably tunable) radio-frequency amplificais, the invention has no application to any tion in which all of the tuning condensers are mechanically interconnected and provided with a single tuning control device. On the other hand, the invention is not necessarily limited inits useful application to the particular purpose just stated. It may, for example, be applied advantageously to radio receivers which do not have unified tuning of these, which has been perhaps the most generally employed, includes the use of a so- I called coupling tube interposed between the To achieve these objects the antenna cirantenna circuit and the first tunable circuit of the amplifier. The alternate prior art practice referred to consists inproviding an antenna circuit which is resonant to a frequency of the order of the highest frequency to be received, and inductively coupling the first wtunable circuit thereto. The latter method or practice involves the use of an antenna coupling transformer having a primary winding of few turns and a large stepup ratio. Under the condition just defined, the antenna circuit has a capacitive reactance throughout the operating frequency range. As a result there is a strong inherent tendency for the antenna circuit to reflect capacity into the first tunable circuit. The reflected device, to add capacity is in effect added in parallel with the capacity of the first tuning condenser. Because of this phenomenon it is generally essential to the attainment of mutual resonance between the several tunable circuits, when they are operated through a common control padding condensers to each of the tunable circuits with the exception of the first one. This, in turn, renders it necessary (in order to cover the whole wavelength range) to substantially increase the maximum capacity of each tuning condenser, thereby incurring added cost. This point will be more fully expounded later.

The capacitive type antenna circuit has a further disadvantage in that the optimum length of aerial is quite critical. Any substantial deviation from the optimum aerial length is apt to result in serious mistlning of the first tunable amplifier circuit due to variation in the reflected capacity. Hence a serious loss of sensitivity as well as selectivity is apt to result.

, Furthermore, since an untunable antenna circuit resonant to a frequency of the order of the highest operating frequency will naturally offer the least impedance to the higher frequencies, and vice versa, and since, as is well known, the ordinar 1 tuned radio-frequencyampl'ifier is most effective at the higher frequencies, it follows that there is a cumulative effect resulting in a very pronounced The coupling tube method which has been mentioned may be said togive very satisfactory resultsunder favorable conditions, but

it is characterized by a serious and apparently unavoidable weakness whenever there are very strong. in'terferingsignal Waves to be contended with. Since neither the antenna circuit nor the input circuit of acoupling tube is tuned to the frequency of the wave to be received, there'is no initial discrimination against interfering waves of other frequencies; and as a consequence it often happens that a coupling tube will act as a detector, due to large grid-potential fluctuations brought about by interfering waves. That being the'case, the wave which it is sought to receive is oftentimes modulated by the interfering wave or waves, and objectionable cross-talk results. The apparent effect is much the same as that due to poor selectivity. The condition referred to is most often experienced with receivers located near powerful transmitting stations. From actual observation it has been ascertained that the above-described phenomenon occurs in a fairly large percentage of instances where coupling tubes are employed, and there is seemingly no way of avoiding it without eliminating the coupling tube.

The present invention is the outcome of extended study and investigation of the problems hereinbefore described. It avoids the use of a coupling tube with its concomitant cross-talk evil and consists, primarily, in the use of an antenna circuit having an inductive instead of a capacitive reactance substantially throughout the range of predetermined operating frequencies. This results in two outstanding advantages, which are: a decidedly more uniform effectiveness with which waves of widely differing frequenciesare received, and a radical diminution of the influence exercised by the antenna C11- cuit upon the mutual resonance of the tuncircuits of the amplifier. On the contrary,'

its effect is to subtract inductance from the first tunable circuit without disturbing its capacity. It would, perhaps, seem on cursory consideration'that the ultimate product in either case would be the same; but there is a very simple and effective remedy available to correct the' latter phenomenon, whereas there is not such an effective remedy available with respect to the former. All that is necessary is to make the actual inductance of the first tunable circuit a little larger than the inductances of the succeeding tunable circuits. Then, when the subtraction of inductance by the antenna circuit occurs, the remaining effective inductance of the first tunable circuit will be equal to that of the other tunable circuits and they will all be in resonance. The inductances not being variable tuning elements, as are the tuning condcnsers, there are no complications involved; and once the proper number of turns for the inductance ,of the first tunable circuit has been determined, the problem is definitely solved. The proper number of turns can easily be determined empirically, or calculated mathematically. This will be explained more fully in connection with the detailed description hereinafter.

With an inductively reactive antenna circuit, as prescribed in accordance with this invention, the dimensions of the aerial are far less critical than in systems heretofore used. This is readily prehensible when it is considered that the reactance of the antenna 9 or less than the probable maximum or mimfin parallelitherewithto offset, respectively, j either of the two 'abnornialornnusual concircuit is mainly inductive, and that the inductance is a fixed quantity determined by the manufacturer of the receiver. For that reason the aerial dimensions and constants can vary considerably without causing a very large percentage variation of antenna circuit reaetance and, consequently, without serious- 1y influencing the first tunable circuit. This does-not mean that the length of the aerial is a matter of no consequence whatever, but it does means that within certain rather wide limits the effect of changes in the aerial length I is relatively v'ery slight;-and those limits are such as to embrace any aerial which experience teaches is likely to be used for broadcast reception.

For the purpose of effectively dealing with the occasional instances where itcmay happen that the aerial-to-ground capacity is greater mum, the invention contemplates and embraces means in the nature of a subordinate feature whereby supplemental capacity of suitable value may be'interposed in the antenna circuit in series with the (inductance or ditions referred to. v

There are broadly two alternative methods of applying to ,the antenna circuit the invention herein described. The first of these, and what is in general considered to be the preferred method, contemplates the employment of a coupling transformer whereof the primary winding which is' included in the antenna circuit has a relatively large number of turns.- In this case substantially all the inductance of the antenna circuit is included in the primary winding of the coupling transformer. The alternate method in this connection involves the use of a coupling transformer with a primary winding having .a relatively small number of turns and a supplemental loading inductance included in the antenna circuit in series with the primary winding of the coupling transformer. "These two alternatives are operatively equivalent, but the former presents some practical advantages over the latter from the standpoint of manufacturing cost and from the further standpoint of the utilization of inherent supplemental capacity coupling which may in many instances be accomplished most economically by the use of a large primary winding, as will bemore precisely explained in connection with the detailed description to follow.

Likewise, with respect to the first tunable circuit to which the antenna circuit iscouplcd, there are two alternative andsubstan ti ally equivalent methods of procedure in applying the invention. One of these contemplates providing on the coupling transformer a secondary winding having alarger actual self-inductance than the desired effective inductance of the first tunable circuit. The alternative in this connection is to provide a secondary winding having a self-inductance which is equal or substantially equal to the desired effective inductance of the circuit and providing a-supplcmental or loading inductance in series therewith of a value equal to the inductance which is subtracted as a result of the association of the antenna circuit with the first tunable circuit A detailed description of certain practicable applications of the invention together with a more extended and enact discussion of its operation and the improved results attainable will now be given with reference to the accompanying drawings in which,

Fig. 1 is a graph comprising two curves representing the voltage gain characteristics of two antenna circuits, one being capacitively reactive and the other inductively reactive: 1. e., the comparatlve effect veness with which incoming signal waves of widely one of which (C) illustrates the voltage gain.

characteristic or overall" amplification throughout a selected range of frequencies with a capacitively reactive invariably tuned antenna circuit, While the other curve (D) illustrates the voltage gain characteristic or overall amplification throughout the same range of frequencies withan inductively reactive antenna circuit in accordance with the present invention;

Fig. 3 is a fragmentary circuit diagram of a vacuum tube radio receiver including a two-stage tuned vacuum tube amplifier and detector togetherwith an antenna circuit in accordance with the preferred embodiment of the present invention; and

Fig. 4 is-a fragmentary circuit diagram illustrating a modified embodiment of the invention.

Inthe graph, Fig. 1, there are two curves A and B, respectively, illustrating'the degree of effectiveness with which wave energy throughout a wide range of frequencies, such as the present broadcast band, is impressed upon the input terminals of the first amplifier tube where the tunable input circuit of the latter is coupled directly to an invariably t-uned antenna. The curve A represents in a general way the variant effectiveness with which wave energy of frequencies between 550 kilocycles and 1500 kilocycles (taken as the broadcast band) is impressed upon the input terminals of the first amplifier tube in an arrangement according to the prior art in which the antenna circuit comprises a primary winding having a small number of turns and is resonant at a frequency of the order ofthe highest frequency in the operating bandthat is to say, a capacitively reactive antenna circuit. Due to the fact that such an antenna circuit presents the lowest series impedance to the highest frequencies intended to be received, it follows as a matter of course that it discriminates vigorously in favor of the high-frequency end of the operating band. And since the succeeding amplifier stages inherently favor the higher frequencies, it is evident that here are two factors operating cumulatively to produce an overall effectiveness of reception at the high-frequency end of theoperating band which is greater than that at the low frequency end. As a matter of fact, the effectiveness at 'the high-frequency end, under the condition stated, is' ordinarily several times as great as the effectiveness at the B of Fig. 1 denotes the salutary change of effectiveness with which the different frequencies are impressed upon the input terminals of the first amplifier tube as the result of using an antenna circuit of low series impedance at the low-frequency end of the operating range, in accordance with the present invention. Here it is seen that the effectiveness with which wave energy of 550 kilocycles is transmitted to the first amplifier tube is considerably greater than the effectiveness at 1500 kilocycles. It is immediately apparent from a comparative consideration of curves A and B that the overall results between the two extremities of frequency are bound to be more nearly uniform when the antenna circuit is resonant at or near the lower end of the frequency band.

The relative increments of signal strength at the two frequency extremes and intermediately, as they appear at the output end of a radio-frequency amplifier with which the present invention is employed are in a considerable measure subject to-the control of the designer, but, nevertheless, dependent somewhat upon the number of stages of radio-frequency amplification unless some special means are utilized to counteract the charv acteristic tendency of radio-frequency amplifiers to accentuate the higher frequencies. For the instant purpose there will be considered only thattype of radio-frequency interstage coupling which is in common use and which is inherently discriminatory in favo pther instance, two factors having opposing tendencies in this respect. Curve C, Fig. 2

low-frequency end. Curve depicts a rapid rise in what is here termed overall amplification, as the operating frequency increases. The term overall amplification, as here employed, does not mean simply the measure of amplification between the input and output ends of a radio-frequency amplifier, but involves in addition the influence of theantenna circuit. Curve C is intended to depict the characteristic conjoint effect of an antenna circuit which discriminates in favor of the higher operating frequencies and a radio-frequency amplifier which does likewise. This is the sort of thing which invariably occurs to a greater or less extent as a direct result of the use of an antenna circuit which is resonant at a frequency of the order of the highest operating frequency in accordance with the practice heretofore followed in the disgn of radio receivers wherein no provision is made for tuning the antenna circuit to the wanted frequency and in which no coupling tubeis employed.

- Curve 1),- Fig. 1-1, is intended to illustrate roughly an example of what can be accomplished in the way of modifying the overall amplification through the use of the present invention. In this case the overall amplification is depicted as being greater at 550 kilocycles than at 1500 kilocycles. This curve has been purposely exaggerated somewhat in order to emphasize the sort of results that can be attained. But since uniformity of effectiveness throughout the operating frequency range is generally the desideratum, the curves of Fig. 9. are not exactly ideal; although curve D is, .by comparison, a fairly close approximation to the ideal. It is evident, however, from these two curves, and from the fact that the results are susceptible of considerable control by the designer, that a fairly close approach-to perfect uniformity throughout the operating frequency range can bereadily accomplished.

Fig. 3 is a circuit diagram illustrating a practicable application of the invention to a vacuum tube radio receiver comprising a two-stage transformer-coupled tuned radiofrequency vacuum tube amplifier and a vacuum tube detector. The audio-frequency or low-frequency portion of the receiver which would ordinarily be coupled to the output diagram.

The radio-frequency amplifier tubes are identified by reference numerals 1 and 2, re-

spectively, while the detector tube is identified by reference numeral 3. Each of the three tubes has a tunable input circuit desig-' nated integrally by reference numerals 4, 5 and 0, respectively. Each tunable circuit includes a variable tuning condenser and an inductanee.the latter constituting the secondary winding of a radio-frequency coupling transformer. The inductances or sectheory of operation.

ondary windings here referred to are designated by reference numerals 7, 8 and 9, respectively, while the variable tuning condensers are designated by reference numerals be assumed that the three tuning condensers are mechanically coupled together either by the use of a common shaft to which allthe condenser rotors are connected, or by any other expedient means, several of which are well known in the art. The mechanical coupling of the three condensers is indicated symbolically bythe connecting lines designated as a whole by-the reference numeral 13.

The output or plate-filament circuit of each of the amplifier tubes 1 and 2 includes the primary windings 14 and 15, respectively, of radio-frequency coupling transformers ofwhich the coils 8 and 9 are the seconda Iies, respectively. It is plain that the inter stage coupling is of the common radio-frequency transformer type" and that'it calls for little discussion beyond what is necessary to complete the description of the invention and explain what is believed to be the correct The amplifier illustrated is arranged to be neutralized in acj,cordance with the well-known Hazeltine method of neutralization, and for this purpose neutralizing condensers 16 and 17 are provided together with auxiliary coils 18 and 19, connected as described in United States Patent No. 1,533,858. The transformer 20 is an audio-frequency transformer having itsprimary winding connected in the toward which the circuit illustrated in Fig.

3 isdirected. Oneof these is to secure a uniform or approximately uniform degree of effec tiveness with which all frequencies within the predetermined operating band or range are caused to appear at theoutput end of the radio freque'ney amplifier, or in other words, at the input terminals of the detector tube. In this respectit may be assumed for the pur pose of comparison that the incoming waves of different frequencies dealt with are of equal amplitude because it is not to be supposed thatawave of small initial amplitude will be received with the same effectiveness as one of large initial amplitude. In other words, it is to be kept in mind that when uniformity of effectiveness or uniformity of overall amplification is referred to herein it is an implied condition precedent that the amplitudes of the received waves as they appear at the antenna, but not affected thereondary windings of't he antenna coupling r transformer. The antenna circuit, furthermore, includes a resistance element 27. This is a damping resistance which may or may not be utilized, as circumstances may dictate. Its function, when employed, is to cut down' the resonance peak of the antenna circuit thereby contributing toward the accomplish ment of uniformity of effectiveness and rendering the length of aerial employed even less critical. By thus decreasing the amplification and broadening the resonance peak, undesired signal voltages at the resonant frequency of the antenna circuitare diminished sufficiently to eliminate from the radiofrequency amplifier cross-talk and forcedoscillations.

It is the intent, in accordance with this invention, that the antenna circuit should have such an inductance that under normal conditions it will be resonant at a fixed frequency of the order of the lowest operating frequency. For example, if the receiver is designed for broadcast reception the lowest operating frequency, according to the present prescribed broadcast band, is 550 kilocycles. The antenna circuit should, therefore, be resonant at or near 550 kilocycles. Under usual conditions it is preferable to make the antenna circuit resonant to a frequency slightly lower than the lowest operating frequency.

the prescribed condition the preferredpra'ctice is to give the primary winding 22 a relaly equivalent results may be obtained through To obtain the necessary inductance-to meet 1 15 tively large number of turns; but substantialthe use of a supplemental inductance inserted in series in the antenna circuit together with a primary winding having a relatively small number of turns. 't'isself-evident that if the antenna circuit 1s resonant at a frequency of the order of the lowest operating frequency it will discriminate in favor .of the lower frequencies and against the higher frequencies,

and that this discrimination will be the more tenna circuit and thence to the input ter minals of the first amplifier tube 1 with considerably greater effectiveness than wave energy of the highest operating frequency. The effectiveness, however, with which wave energy of the lower frequencies is successively amplified and transmitted through the interstage coupling systems is very much less than is the case with energy of the higher 'frequencies. This point has already been fully expounded in connection with the graphs of Figs. 1 and 2-and would seem to require no further discussion.

Now will be considered the effects which the antenna circuit has or may have upon the first tunable circuit 4 and indirectly upon the succeeding tunable circuits 5 and 6. Disregarding for the moment what has been said concerning the antenna circuit 21, and assuming for the purpose of immediate discussion an antenna circuit having little inductance and hence a capacitive re'actance throughout the operating frequency range, it

may be correctly postulated that there will result a reflection of capacity from the antenna circuit into the first tunable circuit 4. The extent to which this will occur depends, in part, upon the degree of coupling between the antenna circuit and the first tunable circuit and, in part, upon the step-up ratio between the primary and secondary windings of the antenna coupling transformer. The smaller the percentage of coupling and the higher the step-up ratio the less will be this reflection. In fact the reflected capacity is inversely proportional to the square of the turns ratio. That is one reason why primary windings of fewturns have heretofore been utilized. But even with such a primary winding it is necessary to resort to very loose coupling if the reflected capacity is to be reduced to such an extent'that it will have no materialinfluence. Such extremely loose coupling introduces an excessive loss of'sensitivity in the receiver. The alternative then,

according to the better engineering practice,

padding condensers connected in' parallel to' the tuning condensers. Thus the minimum capacities of theseveral tunable circuits are increased and equalizedI This means, however, that the maximum capacities of the tuning condensers must be increased over and above what would otherwise be necessary by tal capacity because the ratio of maximum to minimum capacity-(where-a fixed inductance is employed) must equal the square of the ratio of the highest operating frequency to the lowest operating frequency. To illustrate, if the highest operating frequency in the broadcast band is 1500 kilocycles while the lowest operating frequency is 550 kilocycles, the ratio of high to low is roughly 3, and the square of this ratio is 9. This means, for example, that if the reflected capacity amounts to 10 micro-microfarads, the maximum capacity of each tuning condenser would have to be increased micro-microfarads. Due to the need of compensation for reflected capacity, as hereinbefore described, it is not uncommonly necessary to increase the capacity of the tuning condensers as much as 25 or 30 percent over and above what would otherwise be required. This, of course, is a substantial factor in the cost of manufacture, and an important one in low-priced receiving apparatus. v V

If antenna circuits of small inductance, and hence of predominating capacitive reactance, could be relied upon from the standpoint of uniformity, the problem of attaining and maintaining mutual resonance between the tunable circuits under unified control would not be such a serious matter aside from the factor of cost. But such is not the case. It is evident that the reactance in a capacitivetype antenna circuit, when dealing with frequencies of the resent broadcast band, consists for the .ma Or part in the inherent capacity between the aerialand ground; and since this is a factor subject to wide deviation as a result of the use of aerials of indiscriminate lengths, it is apparent that the amount of capacity which will tor. Hence, with such an arrangement (the present invention not being employed) the length of the aerial is amatter of critical importance where it is sought to obtain the best results.

. Let us now consider the performance of an inductively reactive antenna circuit such as that contemplated by this invention, and

be reflected into the first tunable circuit is'a very uncertain and widely variant facillustrated in Fig. 3. In the first place, it

has an effect upon the tunable circuit 4 which -is distinctly different from that of a capacitively reactive antenna circuit. Instead of .refiecting capacity into the circuit 4 it absorbs or subtracts inductance therefrom. This 'is equivalent to reducingthe number of turns of the secondary winding 7. Its immediate influence is, of course, much the same as that resulting from the introduction of reflected capacity in that the mutual resonance between the tunable circuits is adversely aifected. But because of the fact that the alfected'inductance is not the variable tuning element the adverse eifectstated may be easilyremedied by providing additional inductance in the first tunable circuit suflicient to ofiset the reduction'or absorption of inductance therein which is produced by the antenna circuit.

Assuming that the coefiicient of coupling between the primary Winding 22 and the secondary winding 7 is known or determinable, the 'amount of inductance which it would be necessary to add to thetunable'circuit 4 in order to compensate for the aforesaid reduction is readily susceptible of calculation. The relation of actual to effective self-inductance in the first tunable circuit is expressible by the following equation:

In the foregoing equation L, is the actualself-inductance of the tunable circuit 4 as measured with the primary winding 22dissociated therefrom. L is the actual as well face that the denominator 1 /c'-' is always less than unity, but that if K is small, as it should be, the denominator is not much less than unity, and, accordingly, the effective selfinductance of the tunable circuit at is but lit-. tle less than the actual self-inductance. In

The required number of. secondary turns can also be readily determined by trial; and, as a matter of fact, it will usually be necessary to make trial adjustments when the values have been determined by calculation.

Whether the antenna inductance consists entirely of the primary winding 22 or in part of a supplemental inductance in series there with, it is evident that substantially all the inductance of the antenna circuit may form a part of the receiver as produced by the manufacturer, and may have a fixed value.-

It is further evidentthat the inductance constitutesby'far' the major portion of the total antenna reactance. Consequently,v a

' variation of aerial length within any proba-' ble limits of variation will have a relatively small efiect on-the resonant period of the secondary circuit, and, as a consequence, the efi'ect on thetunable circuit 4 will be very slight.

Investigation has shown that the capacities of receiving aerials provided by broadcast listeners range from an approximate minimum of 100 micro-microfarads to an approximate maximum of 300 micro-micrcfiarads.

' These are limits which are not likely to be exceeded in either direction, but may he in some cases. Within the hmits of variations stated, the total antenna reactance variation circuit. Thus, no matter how long the aerial.

might be the total series capacity of the antenna circuit. would be always less than that of the condenser 23.

On the other hand, an aerial might be provided which is entirely too short, and in anticipation of such a condition a parallel actual design it is usually necessary to make fi s iS P This condense! a compromise between optimum amplification and. minimum detuning effect of the calculated actual self-inductance'when the antenna circuit exercises no. influencethereon.

should preferably be of such capacity that the antenna circuit'will have a resonant frequency somewhat lzelow the lowest operating frequency irrespective of how short the aerial may be. Obiiiously the distributed capacity of coil 22 is to be calculated as a part of-shunt capacity 24-. V

The switch25 is shown in a position wherein the condenser 24 is connected in circuit in parallel with primary 22. There are three operating positions of the switch 25. The

two additional positions are: First, that in which neither of-the condensers 23 and 24 .is operatively in circuit; and secondly, that in which the condenser 23 is connected in series with the aerial and theprimary winding 22.

In the latter case condenser 24 is not con-' nccted in circuit. With the arrangement described and illustrated in Fig. 3,110 matter how long or how short the aerial may be its reactance will not vary sufficiently to cause any perceptible mistuning of the tunable circuit 4, providing, of course, that the switch 25 is in the proper position.

An arrangement in accordance with the disclosure of Fig. 3 is capable of being modified by the designer to provide a considerable choice of overall results withirespect to the relative effectiveness of widely different frequencies. Usually, it is preferred to secure, as far as possible, uniform effectiveness at all frequencies within the operating range, but that is notan unexceptionable rule.

The condenser'26 connected as shown in Fig. 3 may be omitted, or may be utilized to supplement the inductive coupling between the antenna circuit and the first tunable circuit to effect a dual coupling therebetween. This condenser may serve to either aid or oppose the inductive coupling as may be desired.-

' condenser 26 will aid the inductively transmitted energy, whereas if wound in the other direction it will oppose. \Vhen the capacity coupling-exactly opposes the magnetic cou- 'pling, the voltage. applied upon the tunable circuit by the condenser is 180 out of phase I with respect to that due to the magnetic cou- 'it does the lower. Therefore, it will .be apparent that the response curve, such as curve D of Fig. 2, can be modified to a considerable extent, i. e., it can be made to slant in either direction or be substantially horizontal, by

. the provision of an ancillary coupling condenser such as condenser .26, in combination with an aidingfor opposing secondary winding, such as inductor 7. Connection 35 extending between the low-potential terminal of primary winding 22 and that of secondary winding 7 provides a return path for current flowing through capacity 26.

Since the amplification in a tuned multistage amplifier progresses geometrically, and

since a transformer-coupled radio-frequency amplifier, such as that illustrated, discrimi-.

.nates in favor of the higher frequencies, it

follows that the larger the number of radioextraordinary number of radio-frequency stages are employed to utilize a coupling condenser such as condenser 26 in opposition to the inductive coupling so as to discriminate against the higher frequencies. lVhether or not the condenser 26 is employed is an arbitrary matter depending largely upon the exigencies of circumstance.

Fig. 4 illustrates an alternative arrangement with respect to that of Fig. 3. In this figure only the antenna circuit and the first amplifier tube are shown, since the remaining portion may be identical with the corresponding portion of Fig. '3.- The antenna circuit in this instance includes two inductance elements 28 and 29 in seriesthe latter being the primary winding of the antenna coupling transformer. The combined inductance of coils 28 and 29 may be the same as the inductance of the primary winding 22 of Fig. 3.

The primary 29 may consist of but afew turns while the coil 28 consists of relatively many turns. The antenna circuit is otherwise identical with that of Fig. 3.

The tunable input circuit 30 of Fig. 4 differs from that of Fig. 3 by the inclusion of a supplemental inductance 31 in series with the secondarygvinding 32. The supplemental inductance 31 may be made equal to the inductance absorbed from the circuit 30 by the antenna circuit. The supplemental inductance 31 may be either fixed or adjustable in value. That is also true of the secondary windings in either instance.

There is sometimes an advantage to be gained through the use of the arrangement of Fig. 3 over that of Fig. 4 in that the adjunctive capacity coupling afforded by condenser 26 may be gained to a sufficient extent through utilization of the inherent capacity between windings. Since the primary winding of Fig. 3 is conceived to be larger than that of Fig. 4 it is possible, other things being equal,

to secure a larger capacity coupling between primary and secondarythrough the inherent a smaller primary winding. Thus with an arrangement otherwise in accordance with Fig. 3, it may be practicable to omit the coupling condenser 26 and still secure the de sired augmentation, whereas the same result might not be susceptible of accomplishment without the coupling condenser 26 in the Fig. 4.

capacity alone than would be possible with Inasmuch as the arrangement of Fig. 4,-

.contemplates'separating the antenna inducisevident that this is likely to lead to an in-' ,sive to enable anyone skilled in" the art to put the invention into practice. But in order to make the disclosure even more complete and specific some further data will be given with v,

respect to a particular example. For this purpose reference is now made to Fig. 3

wherein the following. values may be employed Inductance of primary winding 22 4190 mflli-henries Actual self-inductance of secondary winding 7=. 2013 milli-henries Effective self-inductance of secondary winding 7 2000 milli-hennes Coefficient of coupling between primary winding 22 and secondary 0003 microfarads Capacity of condenser 23. Capacity of condenser 24. 0001 microiarads Capacity of condenser 26 00001 to .00005 microlarads.

There is no need for prescribing values for any of the other elements shown, since these are not in any way predicated upon the present invention and are readily determinable by anyone skilled in the art.

It goes without saying that this invention may be applied in a considerable variety of forms in addition to those specifically -described, and that its scope should be con-. strued accordingly.

What is claimed is:

1. In combination, a capacitive antenna system containing no lumped inductance, and a radio-frequency system for coupling thereto comprising, a circuit tunable by a condenser throughout a frequency range, a transformer having a secondary coil included in said tunable circuit, and a primary coil electromagnetically coupled to said secondary, said vprimary coil being adapted for connection to said antenna system and having an inductance insuring resonance of the antenna system at a frequency fixed below said range.

2. In combination, a capacitive antenna system containing no lumped inductance,- and a radio-frequency system for coupling thereto comprising, a circuit tunable by a condenser throughout a frequency range, a transformer having a secondary coil included in said tunable circuit and a primary coil 'electromagnetically coupled to said second-' ary, said primary coil being adapted" for connection to said antenna system, and an impedance shunted across said primary, said impedance and'primary 0011 together being proportioned to insure resonance of the antenna system at a frequency fixed below said range.

3. In comblnatlon, a capacitive antenna system containing no lumped inductance, and a radio-frequency system for coupling thereto comprising, a circuit tunable by a condenser throughout a frequency range, a transformer having a secondary coil included in said tunablecircuit and a primary coil electromagnetically coupled to said secondary, said primary coil being adapted for con -v neotion to the antenna system and having an inductance insuring resonance of the antenna system at a frequency of the order of the lower frequency limit of said range, and resistance included in the antenna system proportioned to prevent undue response of said system at frequencies approaching and insystem.

4. In 'combination, a capacitive antenna system containing no lumped inductance, and a radio frequency system for coupling thereto comprising, a circuit'includin'g an inductance tunable by a condenser throughout a frequency range, an input path reactively coupled to said tunable circuit, said input path including a coil adapted to be connected to'the antenna system, said coil having an inductance at least as great as the said in-.

ductance of said tunable circuit, and resistance damping means associated with saidcoil and proportioned to prevent undue response cluding the fixed resonance of the antenna of said system at frequencies including and approaching the fixed resonant frequency of the antenna system including said coil. v

5. In a radio-frequency system for coupling to acapacitive; antenna, a circuit tunable by a condenser throughout a frequency range, a coil coupled to said tunable. circuit through means including a condenser, said coil being adapted for connection in the an-' throughout a frequency range, tuning de-- vices therefor having adjustable elements, an antenna system containing no lumped inductance, and means lIlSllIlIlg like extents of movementof the adjustable elements of said devices in tuning from resonance atone frequency to resonance at another frequency comprising, a coil adapted for connection to said antenna system, said coil having an inductance insuring resonance of the antenna system at a frequency ofthe order of the lower limit, of'said range, said coil serving electromagnetically to couple said antenna system to one of said tunable circuits.

7 A radio receiving system comprising cascaded thermionic amplifiers, circultstunable through a frequency range associated with said amplifiers, an antenna system con-' taining no lumped inductance, a first tunable circuit interposed between'said antenna svstem and the firsto f said amplifiers, adjustable condensers for tuning said circuits, and means tending to equalize the overall amplification of said system throughout said range and insuring adjustment of said condensers in unison to like extent comprising a coil adapted for connection to said antenna systern, said coil having an inductance insuring resonance of the antenna system at a frequency of the order of the lower limit of said range, said coil serving electromagnetically 'tocouple said ,antennasystem to said first tunable circuit.

8. In a radio-frequency system for coupling to a capacitive antenna, a circuit tunable by a condenser throughout a frequency range, a transformer having a secondary coil 1ncluded in said tunable circuit and a primary coil adapted to be connected in the antenna circuit, said primary coil having an inductance insuring resonance of the antenna circuit at a frequency of the order of the lower limit of said range and means coupling said i primary coil electrostatically to said secondary coil in aiding phase with its electromagnetic coupling thereto.

9. In a tuned radio-frequency signaling system, a pair of circuits tunable throughout a frequency range including a first and a sec- 0nd coil respectively, like variable condensers for tuning said circuits, an antenna circuit including a coil having an inductance insuring resonance of the antenna circuit at a frequency' of the order of the lower limit ofsaid range, means inductively coupling the antenna circuit to said first coil whereby its eifective inductance is reduced by a certain amount, the lnductance of said first coil 1n the absence of said antenna circuit coil beln 40 larger than the inductance of said second c011 by the same certain'amount, whereby at any glven signal frequency like tuning adjustments of said condensers are secured.

10.111 a radio-frequency system for coupling to an antenna, a circuit tunable by a condenser throughout a frequency range, a

- transformer having a secondary coil included in said tunable circuit and a primary coil electromagnetically coupled to said secondary,

" said primary coil being adapted for connection in the antenna circuit and having its resonant frequency at a frequency fixed below said range.

5 In testimony whereof I aflix my signature.

'WILLIAM A. MAcDONALD. 

